6.4 Chemical Reactions Obj 3; 5 Chemistry

Essential Questions What are the parts of a chemical equation? Why does an equation need to be balanced? How can you tell if an equation is balanced? How do you balance a chemical equation?

Chemical Reactions Chemical reaction is when new substance(s) are formed through the rearrangement of atoms.

Why do reactions occur? Reactions occur to reach a more stable state. Fulfilling the octet rule Reach a more stable energy level

Chemical Equations Representation of a chemical reaction Reactants → Products → means “produces”, “forms”, “yields” etc. There are two types of chemical equations: word and formula

Types of Chemical Equations Word equations list the names of reactants and products Iron metal reacts with oxygen gas to produce iron (III) oxide Chemical equations use chemical formulas to represent reactants and products 4 Fe (s) + 3 O2 (g)→ Fe2O3 (s)

States in Formula Equations Letter in parenthesis indicates state (s) solid (l) liquid (g) gas (aq) aqueous solution (dissolved in water)

Law of Conservation of Mass (conservation of Matter) Mass cannot be created or destroyed; can only change forms All chemical reactions and equations MUST obey this law

Law of Conservation of Mass (cont) The total mass (grams) must be the same on both sides of the equation Ex: 2 H2 + O2 → 2 H2O 4.04 g H2 + 32.00 g O2 → 36.04 g H2O Ex: HCl + RbOH → H2O + RbCl 36.46 g 102.48 g 18.02 g ? Ex: 2 Na + 2 H2O  2 NaOH + H2 91.96 g 72.08 g ? 4.04 g 120.92 g 160 g

Law of Conservation of Mass (cont) The number of atoms of products and reactants must be equal to one another to follow the law of conservation of mass and be balanced.

Identifying Balanced Chemical Equations Identify if the following equations are balanced. H2 + O2 → H2O2 MgS + HCl → H2S + MgCl2 NaHCO3 + H2SO4 → Na2SO4 + CO2 + H2O CH4 + 2 O2 → CO2 + 2 H2O MgO + H2O → Mg(OH)2 H2SO4 + Fe(OH)3 → Fe2(SO4)3 + H2O balanced unbalanced unbal balanced balanced unbalanced

Balancing Steps Write equation with correct symbols and formulas. Count the number of atoms of EACH type of element on each side of the arrow to see if it is balanced. If unbalanced then balance atoms (one at a time) using coefficients (NEVER change the subscripts of a formula, or insert numbers within a formula) Check work by recounting atoms of each element to make sure the sides are equal. Coefficients should be the lowest possible ratio.

Tips for Balancing BrINClHOF – gases that are diatomic or that naturally come in pairs Leave single elements until the end to balance If water is involved balance H and O at the end Polyatomic ions can be count like an element when on both sides

Sample Problem (Not Balanced) Zn + HCl 2 ZnCl2 + H2 (Not Balanced) Adjust the coefficients NOT the subscripts. 1 -Zn- 1 2 -H- 2 2 -Cl- 2 Balanced

Practice Problems Na2O + H2O → NaOH Mg + O2 → MgO CuSO4 + Al → Al2(SO4)3 + Cu C2H4 + O2 → CO2 + H2O

Practice Problems (cont) Na2SO4 + KCl → NaCl + K2SO4 Ca3(PO4)2 + AgNO3 → Ca(NO3)2 + Ag3PO4 Zn(C2H3O2)2 + HBr → ZnBr2 + HC2H3O2 Au2(CrO4)3 + Rb3PO4 → Rb2CrO4 + AuPO4

Essential Questions What are the parts of a chemical equation? Why does an equation need to be balanced? How can you tell if an equation is balanced? How do you balance a chemical equation?

6.4 Tracked Assignment Worksheet